An oxymethylene polymer has excellent mechanical and thermal properties. Particularly, an oxymethylene copolymer has more excellent heat stability and moldability than those of an oxymethylene homopolymer, and therefore has been used widely as an engineering plastic. With respect to the method for producing an oxymethylene copolymer, a method for producing an oxymethylene copolymer by continuous polymerization has been known in which, using a continuous polymerization apparatus comprising a continuous polymerizer that is connected in series with a terminator mixing machine, trioxane and 1,3-dioxolane are subjected to copolymerization using boron trifluoride or a coordination compound thereof as a catalyst, and when the polymerization yield has reached at least 90%, the formed oxymethylene copolymer, which is desirably not subjected to washing step, is contacted with a polymerization terminator in the terminator mixing machine to terminate the polymerization (see, for example, patent document 1).
Patent document 1 discloses the technique for producing an oxymethylene copolymer having suppressed the formation of an unstable portion using as raw materials 1,3-dioxolane and boron trifluoride which can be industrially inexpensively produced and easily handled. Further, this technique is advantageous in that the polymerization yield is high, and that no washing is performed after termination of the polymerization, making it possible to reduce the cost of the monomer recovery.
Patent document 1 has the following description. 1,3-Dioxolane can be easily handled, as compared to the other comonomers, such as ethylene oxide. Further, in the production of an oxymethylene copolymer, when 1,3-dioxolane is used as a comonomer, the base instability is reduced nearly by half, and simultaneously, the crystallization rate is also reduced nearly by half, as compared to those obtained when using ethylene oxide. That is, 1,3-dioxolane and ethylene oxide exhibit totally different behaviors as comonomers of an oxymethylene copolymer.
In this method, however, as the polymerization yield is increased, the formation of an unstable portion having a formate structure easily affected by heat or hydrolysis proceeds. For this reason, the amount of the unstable portion formed is increased in the copolymer at a higher polymerization yield, adversely affecting the polymer quality of a final product, for example, increasing the amount of formaldehyde generated. Therefore, this method is not satisfactory.
There has been known a technique for subjecting trioxane and a comonomer copolymerizable with trioxane to copolymerization in the presence of a cationically active catalyst, wherein, prior to the polymerization, to monomers is added a steric-hindrance phenol having a molecular weight of 350 or more in an amount of 0.001 to 2.0% by weight, based on the total weight of the monomers, and then the copolymerization is conducted (see, for example, patent document 2). Patent document 2 discloses the technique in which trioxane and 1,3-dioxolane are subjected to copolymerization using an ether coordination compound of boron trifluoride as a catalyst in the presence of a steric-hindrance phenol, improving the alkali decomposition rate and weight loss on heating.
Further, there has been known a technique for subjecting trioxane and 1,3-dioxolane to copolymerization using an ether coordination compound of boron trifluoride as a catalyst, wherein the copolymerization is conducted using 1,3-dioxolane having preliminarily added thereto a steric-hindrance phenol having a molecular weight of 350 or more (see, for example, patent documents 3 and 4). However, in all these techniques, the polymerization yield is as low as 85% or less, and further washing is performed simultaneously with termination of the polymerization, and hence a large amount of energy is required for recovering the unreacted monomers, which is disadvantageous from an economical point of view. Therefore, there has been desired the development of a method for producing an oxymethylene copolymer by subjecting trioxane and 1,3-dioxolane to copolymerization using an ether coordination compound of boron trifluoride as a catalyst, which method is advantageous not only in that the polymer quality is improved, for example, the formation of an unstable portion having a formate structure easily affected by heat or hydrolysis is suppressed to reduce the amount of formaldehyde generated, but also in that the polymerization yield is increased.